1. Field of the Invention
The present invention is directed to an X-ray system of the type having an X-ray apparatus carrying an X-ray source and an X-ray receiver that are adjustable relative to a subject to be examined for registering 2D projections of a region of the subject from different projection angles, with subsequent 3D image reconstruction of the subject, and is also directed to an apparatus for the determination of the projection geometries.
2. Description of the Prior Art
X-ray systems of the above type have a C-arm for mounting the X-ray source and the X-ray receiver, this C-arm being seated such in a holder at the X-ray system so that it is motor-adjustable in a specific angular range along its circumference (orbital motion). For acquiring 2D projections from different projection angles for 3D image reconstruction of, for example, a body region of a subject using the C-arm X-ray apparatus, the C-arm--after appropriate placement relative to the subject to be examined--is adjusted along its circumference during the registration of 2D projections of the body region of the subject. 3D images of the body region of the subject are subsequently reconstructed from the 2D projections registered with the X-ray system during the adjustment motion of the C-arm. The 3D reconstruction, however, assumes exact knowledge of the projection geometry, i.e. the knowledge of the position of the X-ray source and the X-ray receiver as well as of the projection angle during the registration of each of the individual 2D projections.
In known stationary X-ray systems and in particular, mobile C-arm X-ray systems, mechanical instabilities occur particularly relating to the adjustment of the C-arm along its circumference, causing deviations of the real adjustment motion of the C-arm from the ideal adjustment motion to occur that make the determination of the projection geometries more difficult.
The following two methods are known for determining the projection geometries.
German OS 195 12 819 discloses the employment of a marker ring, usually formed of plexiglass with inserted metal structures, that is arranged around the body region of the subject to be examined. The metal structures of the marker ring are visible in the 2D projections of the body region to be examined, so that the respective projection geometries of the 2D projections can be calculated from their position. This method, however, has the disadvantage that the marker ring has a relatively large diameter, so that the spacing between X-ray source and the marker ring is very small (a few centimeters) given C-arms with a small diameter. The metal structures are therefore appear greatly enlarged in the 2D projections, so that large parts of the 2D projections are overlaid by the metal structures. Further, only a small region of the metal structures of the marker ring is imaged in the 2D projections, so that the determination of the projection geometries is difficult on the basis of the small number of imaged metal structures.
Calibration measurements can be made before the actual patient measurement, assuming that the system behavior, i.e. essentially the adjustment motions of the C-arm, is reproducible to a high degree. This method, however, is extremely time-consuming and, moreover, can only be applied given a mechanically reinforced, stationary C-arm X-ray apparatus. Application to mobile X-ray apparatus is not possible because of the aforementioned mechanical instability of such an X-ray apparatus. Similar mechanical stabilization is precluded for a mobile X-ray apparatus because of the great increase in weight that would result, which would limit the mobility.
U.S. Pat. No. 5,109,397 discloses a mobile computed tomography system having an X-ray system rotating around a rotation center and including an X-ray source and an X-ray receiver to which sensors are allocated, these sensors moving along with the X-ray system and interacting with a stationary ring allocated to the rotation center for detecting lateral movements of the X-ray system during a scan. The sensors generate signals which are evaluated to determine the spacings between their defined point of attachment and the ring. The acquired data are subsequently utilized in the reconstruction of tomograms. The ring is arranged in the propagation path of an X-ray beam emanating from the X-ray source.
Sensors for determining the distance of two objects from one another are also disclosed in German PS 43 32 254 and German Utility Model 94 08 562. The determination of the distance thereby ensues by measuring the transit time of acoustic waves or electromagnetic waves.
British Specification 1 569 885 describes a computed tomography system with an X-ray apparatus rotating around a rotation center that includes components for determining the projection geometries of the X-ray system during a scan. The components are arranged outside the beam path of the X-ray beam of the X-ray system and include a light source, a photocell and a ring with opaque marks that interact with the light from the light source such that light pulses are generated given rotations of the X-ray system. The projection angles during a scan are determined for the image reconstruction on the basis of the light pulses.
German OS 36 04 955 discloses an X-ray diagnostics apparatus having an image generating system with X-ray radiator and a radiation receiver as well as a patient table. Position sensors in the form of potentiometers that acquire the position of adjustable components of the image generating system are connected to these components.
German OS 195 35 583 also discloses an X-ray diagnostic apparatus with a positioning aid. A light transmitter for emitting a light beam is provided such at an X-ray image intensifier so that this light beam is focused onto a X-ray radiator lying opposite the X-ray image intensifier. In this way, a positioning of the X-ray radiator and X-ray image intensifier can ensue with reference to an examination subject without emitting X-rays.